The present invention relates to high-frequency dielectric heating using a magnetron such as a microwave oven, and more particularly to a step-up transformer for driving a magnetron by a switching power source.
FIG. 6 is a diagram showing the structure of a magnetron driving power source using a step-up transformer intended for the invention.
In FIG. 6, an alternating current sent from a commercial power source 11 is rectified into a direct current by a rectifying circuit 13, and the direct current is smoothened by a choke coil 14 and a filter capacitor 15 on the output side of the rectifying circuit 13 and is given to the input side of an inverter 16. The direct current is converted to have a desirable high frequency (20 kHz to 40 kHz) by turning ON/OFF a semiconductor switching unit in the inverter 16.
The inverter 16 includes a switching unit group having two power IGBTs 161 and 162 switching a direct current at a high speed and connected in series, for example, and an inverter control circuit 165 for driving the switching unit group.
A series connecting circuit for the power IGBT is connected between both positive and negative terminals of the direct current, and similarly, a series connecting circuit including two capacitors 163 and 164 is also connected between both positive and negative terminals of the direct current. Both ends of a primary winding 181 of a step-up transformer 18 are connected between a connecting point P1 of the power IGBTs and a connecting point P2 of the capacitors, respectively.
Furthermore, the gate of the power IGBT is driven by the inverter control circuit 165 and a current flowing to the primary side of the step-up transformer 18 is switched to ON/OFF at a high speed.
A signal input to the inverter control circuit 165 detects the primary side current of the rectifying circuit 13 by a CT 17, and the detected current is input to the inverter control circuit 165 and is used for controlling the inverter 16.
In the step-up transformer 18, a high-frequency voltage to be the output of the inverter 16 is applied to the primary winding 181 and a high voltage corresponding to a winding ratio is obtained from a secondary winding 182.
Moreover, a winding 183 having the small number of winds is provided on the secondary side of the step-up transformer 18 and is used for heating a filament 121 of a magnetron 12. The secondary winding 182 of the step-up transformer 18 includes a voltage doubler half-wave rectifying circuit 19 for rectifying an output thereof.
The voltage doubler half-wave rectifying circuit 19 is constituted by a high-voltage capacitor 191 and two high-voltage diodes 192 and 193, and the high-voltage capacitor 191 and the high-voltage diode 192 are conducted in a positive cycle (for example, the upper end of the secondary winding 182 is set to be positive in the drawing) and the left and right plates of the high-voltage capacitor 191 are charged to be positive and negative respectively in the drawing. Next, the high-voltage diode 193 is conducted in a negative cycle (the lower end of the secondary winding 182 is positive) and a double voltage obtained by adding the voltage of the high-voltage capacitor 191 charged in advance to that of the secondary winding 182 is applied between an anode 122 and the cathode 121 in the magnetron 12.
It is also possible to constitute a voltage doubler full-wave rectifying circuit by two high-voltage capacitors and two high-voltage diodes in place of the voltage doubler half-wave rectifying circuit 19. This is preferable in that the peak of an anode current flowing to the magnetron can be reduced and a durability can be enhanced in a high output.
While an example of the magnetron driving power source using the step-up transformer intended for the invention has been described above, the driving power source is not restricted thereto but any driving power source including a transformer for boosting a high frequency may be employed.
With the needs of a reduction in the size of a microwave oven, it is necessary to reduce the size of a step-up transformer. Therefore, a high frequency has been used as described above in place of a low frequency. For the core of the transformer, a metal core which is advantageous to a reduction in a size, a saturation and a cost (amorphous, a silicon steel plate) has been used at a low frequency. However, the metal core has not been used because of a great high-frequency loss at a high frequency. Instead, a ferrite core has been used.
There has been known a step-up transformer in which two ferrite cores are used to be butted each other in a gap, as shown in JP-A-2001-015259 (Japanese Patent Application Publication Number: 2001-015259), JP-A-2002-134266 (Japanese Patent Application Publication Number: 2002-134266) and JP-A-2001-189221 (Japanese Patent Application Publication Number: 2001-189221).
FIG. 7A and FIG. 7B show an example of a conventional well-known step-up transformer using a ferrite core, FIG. 7A being a longitudinal sectional view and FIG. 7B being a view seen in a direction of X—X of FIG. 7A. For easy understanding, a winding portion is omitted in FIG. 7B.
In FIG. 7A, 18′ denotes a step-up transformer, 181′ denotes a primary winding, 182′ denotes a secondary winding, 183′ denotes a heater winding and 184′ denotes a coil bobbin.
18A′ and 18B′ denote U-shaped ferrite cores (circular sections), A1′ denotes a core (a middle core) positioned in the winding in the core constituting the U-shaped ferrite core 18A′, A3′ denotes an outer core provided on the outside of the winding in the core constituting the U-shaped ferrite core 18A′ and positioned in parallel with the middle core A1′, and A2′ denotes a coupling core for coupling the middle core A1′ to the outer core A3′. Similarly, B1′ denotes a core (a middle core) positioned in the winding of the core constituting the U-shaped ferrite core 18B′, B3′ denotes an outer core provided on the outside of the winding in the core constituting the U-shaped ferrite core 18B′ and positioned in parallel with the middle core B′, and B2′ denotes a coupling core for coupling the middle core B1′ to the outer core B3′.
The primary winding 181′, the secondary winding 182′ and the heater winding 183′ are disposed in parallel on the same axis where the middle core A1′ and the middle core B1′ are opposed to each other. In case of a power source for driving a magnetron which often treats a large power, the use of a zero-volt switching method (hereinafter referred to as a ZVS method) based on a voltage resonance is a mainstream in order to relieve the load of a power semiconductor. In the ZVS method, it is necessary to set the coupling coefficient of the step-up transformer to be approximately 0.6 to 0.85 in order to obtain a resonance voltage, and a gap G′ is provided.
The sectional area of the outer core A3′ is almost equal to or slightly smaller than that of the middle core A1′ (70% or less) as seen from FIG. 7B.
An installation area for attachment to a printed board is represented as L1′×L2′ in case of such a conventional step-up transformer, wherein a full length (including a gap) in an axial direction of the middle core A1′ and the middle core B1′ is represented by L1′ and a length from the outer end of the coil bobbin 184′ to the outer core A3′ (B3′) in the U-shaped ferrite core 18A′ is represented by L2′.
It is necessary to more increase a peak current flowing to the primary side of the step-up transformer when further raising the output of the magnetron. Consequently, the size of the step-up transformer is inevitably increased so that an installation area thereof is also increased.